To do any further troubleshooting, you'll likely need a signal tracer. If you were in the US, I'd recommend the Radio Shack mini-amplifier but I doubt it's available there. Maybe you can find something similar.

The mini amp would be used to listen to the signals on the input and output pins of several components (ICs, transistors...) to see if you can find the point where the noise is entering the audio chain. If the problem is internal to the head unit, it's possible that the mini-amp will help track it down but there are no guarantees.

Power up the head unit in the vehicle (assuming that the noise was being produced by the charging system in the vehicle) and connect the mini-amp to each channel to confirm that the same channel is noisy with the mini-amp as with the original amplifier.